The avian progesterone receptor is being used as a model for studying the role of phosphorylation in receptor structure and function. This receptor has been shown to be phosphorylated at multiple sites on serine residues and the extent of phosphorylation is rapidly increased following progesterone treatment. The present studies are designed to identify the actual sites of phosphorylation using biochemical techniques. In addition, studies will be initiated to reveal the structural or functional significance of phosphorylation through the analysis of mutant receptor forms. Intact receptor and a series of deletion mutants will be expressed in cultured cells. These will be compared with regard to hormone-dependent phosphorylation, physical properties, and biological activity. Eventually, the phosphorylation sites will be eliminated, individually or as a group, by use of serine replacement mutants. Such studies should provide a much more complete description of phosphorylation plus important clues on the biological significance of this event.